Pluralities of measures are possible in order to achieve high degrees of thermal efficiency. One of the measures would be to increase the inflow temperatures of the steam flowing into the thermal turbomachine, in particular a steam turbine. At present, efforts are being made to increase the steam inflow temperature to up to 700° C. or even higher.
Such high steam inflow temperatures require a specific selection of materials which withstand the thermal loading. According to current findings, nickel-based materials are suitable for high steam inflow temperatures. However, this material is many times more expensive than customary materials.
In thermal turbomachines, for example steam turbines, the rotor and the housing, in particular the inner housing, are subjected to thermal loading. The housings of steam turbines are usually designed with two shells. In this case, the inner housing contains the portion of steam expansion, where the highest thermal loading occurs, and comparatively cooler steam, e.g. the waste steam, flows around this inner housing and is absorbed again by the outer housing. The outer housing is arranged around the inner housing.
The inner housings are designed as cast structures, i.e. they are as it were produced from a casting, even though only the one flow region has to withstand the high thermal loading. A material which withstands the thermal loading and is then used for the entire inner housing is often selected. However, this is not optimal in terms of cost since comparatively highly heat resistant materials are used for regions which are subjected to less thermal loading and where comparatively low temperatures prevail. Comparatively inexpensive materials which are not so highly heat resistant can be used at these locations.
The manufacturing limits for nickel-based materials mean that the weight of the inner housing is problematic for future steam turbines which are to be suitable for steam inflow temperatures of 700° C., since it may prove to be that housings such as these can no longer be cast owing to their weight.
A further problem with inner housings such as these is warping, which occurs during opening after a specific operating period, e.g. during a major overhaul. This warping occurs as a result of high temperature differences over the wall thickness owing to the intended cooling effect. Such distortion can be observed, in particular, in the inflow region of the inner housing. The distortion results in thermal stresses.
EP 1 033 478 discloses a housing which is formed from various materials which are axially welded to one another.
It is known from EP 1 586 394 to form regions of components which are resistant to loading with an additional material in order to increase the resistance.